We show how we can transform the ${\mathscr H}_\infty$ and ${\mathscr H}_2$ control problems of descriptor systems with invariant zeros on the extended imaginary into problems with state-space systems without such zeros. Then we present necessary and sufficient conditions for existence of solutions of the original problems. Numerical algorithm for ${\mathscr H}_\infty $ control is given, based on the Nevanlinna-Pick theorem. Also, we present an explicit formula for the optimal ${\mathscr H}_2$ controller.
parametrization of stabilizing controllers, inner matrices, ${\mathscr H}_\infty $ and ${\mathscr H}_2$ control
93D15, 49J15